Abstract:

A computer-implemented method includes accessing a set of multicomponent
marine noise data exhibiting a plurality of polarization vectors at each
of a plurality of co-located pressure and particle motion data points on
a marine seismic survey apparatus; and determining a set of perturbation
noise data for the marine seismic survey apparatus from the polarization
vectors. Computer readable program storage media are encoded with
instructions that, when executed by a processor, perfume the
computer-implemented method. One computing apparatus is programmed to
perform the computer-implemented method.

Claims:

1. A computer-implemented method, comprising:accessing a set of
multicomponent calibration data exhibiting a plurality of polarization
vectors at each of a plurality of paired pressure and particle motion
data points on a marine seismic survey apparatus;determining a set of
perturbations for the marine seismic survey apparatus from the
polarization vectors; anddetermining a set of calibration values
corresponding to the perturbations.

2. The computer-implemented method of claim 1, wherein accessing the
multicomponent calibration data includes accessing a set of acquired
multicomponent calibration data.

3. The computer-implemented method of claim 1, wherein accessing the
multicomponent calibration data includes accessing a set of acquired and
interpolated multicomponent calibration data.

4. The computer-implemented method of claim 1, further comprising
interpolating data from measurements acquired through paired sensors to
generate a plurality of data points to generate at least a portion of the
calibration data.

7. The computer-implemented method of claim 1, further comprising
mitigating the determined perturbations in a set of marine seismic survey
data.

8. The computer-implemented method of claim 7, wherein mitigating the
determined perturbation noise data in the seismic survey data includes
removing the determined perturbations from the seismic survey data.

9. The computer-implemented method of claim 8, wherein removing the
determined perturbation noise data from the seismic survey data includes
calibrating the acquired data to correct the deviation of the response of
the sensor from the nominal response.

10. The computer-implemented method of claim 7, wherein mitigating the
determined perturbation noise data in the seismic survey data includes
calibrating the acquired data to correct the deviation of the response of
the sensor from the nominal response.

12. A program storage medium, encoded with instructions that, when
executed by a computing device, perform a method comprising:accessing a
set of multicomponent calibration data exhibiting a plurality of
polarization vectors at each of a plurality of paired pressure and
particle motion data points on a marine seismic survey
apparatus;determining a set of perturbations for the marine seismic
survey apparatus from the polarization vectors; anddetermining a set of
calibration values corresponding to the perturbations.

13. The program storage medium of claim 12, wherein accessing the
multicomponent calibration data includes accessing a set of acquired
multicomponent calibration data.

14. The program storage medium of claim 12, wherein accessing the
multicomponent calibration data includes accessing a set of acquired and
interpolated multicomponent calibration data.

15. The program storage medium of claim 12, wherein the method further
comprises interpolating data from measurements acquired through paired
sensors to generate a plurality of data points to generate at least a
portion of the calibration data.

16. The program storage medium of claim 12, wherein determining the
perturbations includes relating the polarization vectors to physical
model parameter errors.

17. The program storage medium of claim 12, the method further comprises
mitigating the determined perturbations in a set of marine seismic survey
data.

18. The program storage medium of claim 12, wherein the paired pressure
and particle motion data points are co-located.

19. A programmed computing apparatus, comprising:a processor;a bus
system;a storage communicating with the processor over the bus system;
anda software component that, when invoked by the processor, performs a
method including:accessing a set of multicomponent calibration data
exhibiting a plurality of polarization vectors at each of a plurality of
paired pressure and particle motion data points on a marine seismic
survey apparatus;determining a set of perturbations for the marine
seismic survey apparatus from the polarization vectors; anddetermining a
set of calibration values corresponding to the perturbations.

20. The programmed computing apparatus of claim 19, wherein accessing the
multicomponent calibration data includes accessing a set of acquired
multicomponent calibration data.

21. The programmed computing apparatus of claim 19, wherein accessing the
multicomponent calibration data includes accessing a set of acquired and
interpolated multicomponent calibration data.

22. The programmed computing apparatus of claim 19, wherein the method
further comprises interpolating data from measurements acquired through
paired sensors to generate a plurality of data points to generate at
least a portion of the calibration data.

26. A method, comprising:acquiring a set of calibration data for a marine
survey apparatus including paired pressure and particle motion sensors,
the set acquisition including:imparting a plurality of seismic signals at
different times and from different depths in a marine environment;
andacquiring a set of calibration data from reflections of the seismic
signals at the paired pressure and particle motion sensors;performing a
marine seismic survey using the paired pressure and particle motion
sensors to acquire a set of seismic survey data; andmitigating the
perturbations in the seismic survey data, including:estimating the
perturbations on the response of the marine survey apparatus by using the
acquired calibration signal;determining the calibration values on the
response of the marine survey apparatus to undo the perturbations;
andremoving the estimated perturbations on the response of the marine
survey apparatus.

27. The method of claim 26, wherein accessing the multicomponent marine
seismic data includes accessing a set of acquired multicomponent marine
seismic data.

28. The method of claim 26, wherein accessing the multicomponent marine
seismic data includes accessing a set of acquired and interpolated
multicomponent marine seismic data.

29. The method of claim 26, further comprising interpolating data from
measurements acquired through paired sensors to generate a plurality of
data points to generate at least a portion of the noise data.

30. The method of claim 26, wherein determining the perturbations includes
relating the polarization vectors to physical model parameter errors.

32. The method of claim 26, wherein determining the perturbations include
relating the polarization vectors to physical model parameter errors.

33. The method of claim 26, wherein removing the determined perturbation
noise data from the seismic survey data includes calibrating the acquired
data to correct the deviation of the response of the sensor from the
nominal response..

34. A method, comprising:estimating a set of perturbations on the response
of a marine survey apparatus including paired pressure and particle
motion sensors,imparting a plurality of seismic calibration signals at
different times and from different depths in a marine
environment;acquiring a set of reflections of the seismic calibration
signals at a plurality of paired pressure and particle motion
sensors;relating the polarization parameters of the acquired set of
seismic calibration signals to the perturbations on the response of the
seismic survey apparatus; andestimating the perturbations and calibration
values based on the relationship;performing a marine seismic survey using
the paired pressure and particle motion sensors to acquire a set of
seismic survey data, andmitigating the perturbation errors in the seismic
survey data.

35. The method of claim 34, wherein mitigating the perturbation noise in
the seismic survey data includes: correcting the response of the marine
survey apparatus to remove the estimated perturbations on the response of
the marine survey apparatus.

36. The method of claim 34, wherein imparting a plurality of seismic
calibration signals included imparting a plurality of seismic signals.

37. A marine seismic surveying apparatus, comprising;a seismic survey
vessel;at least one seismic source capable of imparting a plurality of
seismic calibration signals at different times and from different depths
in a marine environment;a plurality of seismic cables including a
plurality of paired pressure and particle motion sensors distribute along
the length thereof and capable of acquiring a set of reflections of the
seismic calibration signals at the paired pressure and particle motion
sensors; anda computing apparatus aboard the seismic survey vessel and
capable of:accessing a set of multicomponent calibration data exhibiting
a plurality of polarization vectors at each of a plurality of paired
pressure and particle motion data points on a marine seismic survey
apparatus;determining a set of perturbations for the marine seismic
survey apparatus from the polarization vectors; anddetermining a set of
calibration values corresponding to the perturbations.

[0004]This invention disclosure relates to marine seismic surveying, and,
in particular, to estimation and correction of perturbations on seismic
particle motion sensors in such a survey.

[0005]2. Discussion of Related Art

[0006]This section of this document is intended to introduce various
aspects of the art that may be related to various aspects of the present
invention described and/or claimed below. This section provides
background information to facilitate a better understanding of the
various aspects of the present invention. As the section's title implies,
this is a discussion of related art. That such art is related in no way
implies that it is also prior art. The related art may or may not be
prior art. It should therefore be understood that the statements in this
section of this document are to be read in this light, and not as
admissions of prior art.

[0007]Seismic exploration involves surveying subterranean geological
formations for hydrocarbon deposits. A survey typically involves
deploying seismic source(s) and seismic sensors at predetermined
locations. The sources impart acoustic waves into the geological
formations. Features of the geological formation reflect the pressure
waves to the sensors. The sensors receive the reflected waves, which are
detected, conditioned, and processed to generate seismic data. Analysis
of the seismic data can then indicate the presence or absence of probable
locations of hydrocarbon deposits.

[0008]Some surveys are known as "marine" surveys because they are
conducted in marine environments. Note that marine surveys may be
conducted not only in saltwater environments, but also in fresh and
brackish waters. Marine surveys come in at least two types. In a first
type, an array of seismic cables (known as "streamers") and seismic
sources is towed behind a survey vessel. In a second type, an array of
seismic cables (known as "ocean bottom cables"), each of which includes
multiple sensors, is laid on the ocean floor, or seabed, and a seismic
source is towed from a survey vessel.

[0009]Historically, towed array, marine seismic surveys only employed
pressure waves and the receivers detected any passing wavefront. This
includes two types of wavefronts. The first are those reflected upward to
the receivers from the geological formation. The second are those that
are reflected downward from the surface of the water. The upward
reflections are desirable because they generally contain information
about the geological formation under survey. The downward reflections are
undesirable because they interfere with the upward reflections and reduce
the bandwidth of the seismic signal.

[0010]The art has therefore recently begun moving to "multicomponent"
surveys in which, for example, not only is the passing of a wavefront
detected, but also the direction in which it is propagating. Knowledge of
the direction of travel permits determination, for instance, of which
wavefronts are traveling upward and which are traveling downwards.
Multicomponent towed-array surveys include a plurality of receivers that
detect not only the pressure wave, but also the velocity, or time
derivatives (e.g., acceleration) thereof, of the passing wavefront. These
receivers will hereafter be referred to as "particle motion sensors"
because they measure the velocity or acceleration of displaced particles.
The pressure sensor is typically a hydrophone, and the particle motion
sensors are typically geophones or accelerometers.

[0011]However, multicomponent surveys are more sensitive to what may be
called "perturbations". One kind of perturbation, for example, is what is
known as an "alignment perturbation". Sensors in a streamer that form a
part of a towed array are frequently oriented in an orthogonal x-y-z
coordinate system in which the x-y-z axes are defined as in-line with the
streamer, cross-line to the streamer, and in depth. In an alignment
perturbation, the sensor is misaligned relative to the streamer such that
one or more of its x-y-z axes is out of alignment with the corresponding
in-line, cross-line, and depth axes of the streamer. This is but one
example of a perturbation, and there are others. Another kind, for
example, pertains to amplitude sensitivities.

[0012]Perturbations are undesirable because they also lead to errors in
the seismic data that is acquired in the survey. Errors in the data, in
turn, can lead to errors in the analysis for the location of the
hydrocarbon deposits. Those in the art have therefore begun to develop
techniques by which this error can be eliminated, or at least mitigated.

[0013]The present invention is directed to overcoming, or at least
reducing the effects of, one or more of the problems set forth above.

BRIEF SUMMARY OF THE INVENTION

[0014]In a first aspect, the invention is a computer-implemented method,
comprising: accessing a set of multicomponent marine calibration data
exhibiting a plurality of polarization vectors at each of a plurality of
paired pressure and particle motion data points on a marine seismic
survey apparatus; determining a set of perturbations for the marine
seismic survey apparatus from the polarization vectors; and determining a
set of calibration values corresponding to the perturbations.

[0015]In a second aspect, the invention includes a method, comprising:
acquiring a set of calibration data for a marine survey apparatus
including paired pressure and particle motion sensors; performing a
marine seismic survey using the paired pressure and particle motion
sensors to acquire a set of seismic survey data; and mitigating the
perturbation noise in the seismic survey data. Acquiring the calibration
data includes: imparting a plurality of seismic signals at different
times and from different depths in a marine environment; and acquiring a
set of calibration data from reflections of the seismic signals at the
paired pressure and particle motion sensors. Mitigating the perturbations
includes: estimating the perturbations on the response of the marine
survey apparatus by using the acquired calibration signal; determining
the calibration values on the response of the marine survey apparatus to
undo the perturbations; and removing the estimated perturbations on the
response of the marine survey apparatus.

[0016]In a third aspect, the invention is a method, comprising: estimating
a set of perturbations on the response of a marine survey apparatus
including paired pressure and particle motion sensors, performing a
marine seismic survey using the paired pressure and particle motion
sensors to acquire a set of seismic survey data, and mitigating the
perturbation errors in the seismic survey data. Estimating the
perturbations includes: imparting a plurality of acoustic calibration
signals at different times and from different depths in a marine
environment; acquiring a set of reflections of the acoustic calibration
signals at a plurality of paired pressure and particle motion sensors;
relating the polarization parameters of the acquired set of acoustic
calibration signals to the perturbations on the response of the seismic
survey apparatus; and estimating the perturbations and calibration values
based on the relationship.

[0017]In a fourth aspect, the invention includes a marine seismic
surveying apparatus, comprising; a seismic survey vessel; at least one
seismic source capable of imparting a plurality of seismic calibration
signals at different times and from different depths in a marine
environment; a plurality of seismic cables including a plurality of
paired pressure and particle motion sensors distribute along the length
thereof and capable of acquiring a set of reflections of the seismic
calibration signals at the paired pressure and particle motion sensors;
and a computing apparatus aboard the seismic survey vessel. The computing
apparatus is capable of: accessing a set of multicomponent calibration
data exhibiting a plurality of polarization vectors at each of a
plurality of paired pressure and particle motion data points on a marine
seismic survey apparatus; determining a set of perturbations for the
marine seismic survey apparatus from the polarization vectors; and
determining a set of calibration values corresponding to the
perturbations.

[0018]In other aspects the invention includes computer readable program
storage media encoded with instructions that, when executed by a
processor, perfume the software implemented aspects of the invention and
computing apparatus programmed to perform those aspects.

[0019]The above presents a simplified summary of the invention in order to
provide a basic understanding of some aspects of the invention. This
summary is not an exhaustive overview of the invention. It is not
intended to identify key or critical elements of the invention or to
delineate the scope of the invention. Its sole purpose is to present some
concepts in a simplified form as a prelude to the more detailed
description that is discussed later.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0020]The invention will hereafter be described with reference to the
accompanying drawings, wherein like reference numerals denote like
elements, and:

[0021]FIG. 1 illustrates one particular embodiment of a method practiced
in accordance with one aspect of the present invention;

[0022]FIG. 2 depicts a computing apparatus on which one particular
embodiment of the present invention may be practiced;

[0023]FIG. 3A-FIG. 3c depict a towed array, marine seismic survey by which
seismic data may be acquired in one embodiment of a first aspect of the
invention;

[0031]FIG. 12 conceptually illustrates the interpolation of data from the
location of acquisition to the position of a paired sensor in some
alternative embodiments;

[0032]FIG. 13A and FIG. 13B depicts a seabed survey with which the present
invention may be practiced in some alternative embodiments.

[0033]While the invention is susceptible to various modifications and
alternative forms, specific embodiments thereof have been shown by way of
example in the drawings and are herein described in detail. It should be
understood, however, that the description herein of specific embodiments
is not intended to limit the invention to the particular forms disclosed,
but on the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the scope of the invention
as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

[0034]One or more specific embodiments of the present invention will be
described below. It is specifically intended that the present invention
not be limited to the embodiments and illustrations contained herein, but
include modified forms of those embodiments including portions of the
embodiments and combinations of elements of different embodiments as come
within the scope of the following claims. It should be appreciated that
in the development of any such actual implementation, as in any
engineering or design project, numerous implementation-specific decisions
must be made to achieve the developers' specific goals, such as
compliance with system-related and business related constraints, which
may vary from one implementation to another. Moreover, it should be
appreciated that such a development effort might be complex and time
consuming, but would nevertheless be a routine undertaking of design,
fabrication, and manufacture for those of ordinary skill having the
benefit of this disclosure. Nothing in this application is considered
critical or essential to the present invention unless explicitly
indicated as being "critical" or "essential."

[0035]The words and phrases used herein should be understood and
interpreted to have a meaning consistent with the understanding of those
words and phrases by those skilled in the relevant art. No special
definition of a term or phrase, i.e., a definition that is different from
the ordinary and customary meaning as understood by those skilled in the
art, is intended to be implied by consistent usage of the term or phrase
herein. To the extent that a term or phrase is intended to have a special
meaning, i.e., a meaning other than that understood by skilled artisans,
such a special definition will be expressly set forth in the
specification in a definitional manner that directly and unequivocally
provides the special definition for the term or phrase.

[0036]The present invention includes a technique for estimating and
correcting perturbations in multicomponent seismic data acquired in a
marine survey. The technique includes, in various aspects and
embodiments, a computer-implemented method, various apparatuses for use
in implementing the method, and a seismic data set in which perturbation
error has been mitigated through performance of the method.

[0037]The present invention, more particularly, is directed to a technique
by which acquired seismic data can be corrected for perturbations. The
effect of these perturbations in the acquired seismic data can be
described as noise since it interferes with the information that actually
describes the geological formation under survey. Still more particularly,
the present invention describes a technique for estimating and mitigating
perturbations in a multicomponent seismic data including co-located
pressure and particle motion measurements.

[0038]The technique relates the perturbation noise to physical model
parameters such as sensor sensitivities and sensor alignment misalignment
angle with respect to cable axis. The components and the magnitude of the
polarization vectors are then related to an acquired calibration signal
with particle motion and pressure sensors. The perturbations are then
estimated by using the measured polarization information from a plurality
of seismic sources that are shot while the seismic streamer is towed by a
seismic vessel.

[0039]As used herein, the "perturbation noise" is the difference between
the recorded seismic data in the absence of any perturbations and the
recorded seismic data in the presence of physical parameter model errors.
In this context, the "perturbations" are the deviations of physical
parameters of the sensor from their nominal values. These include
sensitivity and alignment perturbations as mentioned above. Hence,
perturbation noise can be quantified as a function of sensitivities and
misalignment angles. The signals measured by pressure and particle motion
sensors are functions of magnitude of the polarization vector and the
angles between the components of the polarization vector. The
sensitivities of the sensor can therefore be estimated from magnitude of
the polarization vector and alignment of the sensors can be estimated
from the direction of the polarization vector. The presence of co-located
data points from pressure sensors and particle motion sensors in a
multicomponent streamer allows estimation of sensor sensitivities
independent of sensor alignment. Because the new technique uses signal
records for correction of perturbation errors, it may be referred to as a
Signal-based Perturbation Estimation and Correction ("SPEC") technique.

[0040]The present invention will now be described with reference to the
attached figures. Various structures, systems and devices are
schematically depicted in the drawings for purposes of explanation only
and so as to not obscure the present invention with details that are well
known to those skilled in the art. Nevertheless, the attached drawings
are included to describe and explain illustrative examples of the present
invention.

[0041]Turning now to the drawings, wherein like reference numbers
correspond to similar components throughout the several views, FIG. 1
presents a computer-implemented method 100 that may be performed on an
apparatus such as the computing apparatus 200 of FIG. 2 in the mitigation
of perturbation error in multicomponent marine seismic survey data. The
technique disclosed herein develops a set of calibration data for
characterization of perturbation noise in marine seismic survey data. The
calibration data is acquired in such a manner that it exhibits a
plurality of polarization vectors at each of a plurality of co-located
pressure and particle motion data points. The perturbations are estimated
from the calibration data acquired in, for example, the manner
illustrated in FIG. 3A-FIG. 3c and discussed further below. Once the
perturbations are estimated, the response of the marine survey apparatus
can be calibrated to mitigate the perturbation noise on seismic survey
data.

[0042]FIG. 2 shows selected portions of the hardware and software
architecture of a computing apparatus 200 such as may be employed in some
aspects of the present invention. The computing apparatus 200 includes a
processor 205 communicating with storage 210 over a bus system 215. The
storage 210 may include practically any type of medium, including a hard
disk and/or random access memory ("RAM") and/or removable storage such as
a floppy magnetic disk 217 and an optical disk 220.

[0043]The storage 210 is encoded with the acquired seismic survey data
225. The acquired seismic survey data 225 is "multicomponent" data and
includes, as is shown in FIG. 2, both pressure data 227 (i.e., "P data")
and particle motion data 228 (e.g., "Z data"). The storage 210 is
furthermore encoded with calibration data 231, which is also
"multicomponent" data and is similarly comprised of pressure data 232 and
particle motion data 233. The calibration data 231 is acquired as
described further below in a manner such that it exhibits a plurality of
polarization vectors at each of a plurality of co-located pressure and
particle motion data points.

[0044]Note that the acquired seismic survey data 225 and calibration data
231 are digital at the time they are stored on the storage 210. In the
particular embodiments disclosed herein, the acquired seismic survey data
225 and calibration data 231 are digital at the point of acquisition.
However, the point of digitization may vary depending on the
implementation. The data may therefore be digital upon output from the
sensors (not shown) by which it is acquired or upon conversion after
output and prior to storage.

[0045]The acquired seismic survey data 225 and calibration data 231 may be
stored using any suitable data structure known to the art. The data
structure will typically be, for example, a flat file or a text delimited
file. However, acceptable alternatives include structures such as a
database, a list, a tree, a table, etc. The invention is not limited by
the manner in which the acquired seismic survey data 225 and calibration
data 231 are stored.

[0046]The storage 210 is also encoded with an operating system 230, user
interface software 235, and an application 265. The user interface
software 235, in conjunction with a display 230 and peripheral I/O
devices such as a keypad or keyboard 250, a mouse 255, or a joystick 260,
implements a user interface 245. The processor 205 runs under the control
of the operating system 230, which may be practically any operating
system known to the art. The application 265, when invoked, performs the
method of the present invention, e.g., the method 100 of FIG. 1. The user
may invoke the application in conventional fashion through the user
interface 245.

[0047]Referring now to both FIG. 1 and FIG. 2, the method 100 is a
computer-implemented method for determining perturbations--represented by
the perturbation data 236--and the corresponding calibration values 237
associated with a given marine survey apparatus. The calibration values
237 can then, in another aspect of the invention, be used to calibrate
the response of the marine survey apparatus to mitigate perturbation
errors in the marine seismic survey data 225. In the illustrated
embodiment, the method 100 is performed by the execution of the
application 265 by the processor 205 under the control of the OS 230, all
shown in FIG. 2 and discussed above. Note, however, that the invention is
not limited by the nature of the software component by which the method
is implemented. In alternative embodiments, for example, the method 100
may be implemented in, e.g., a utility or some other kind of software
component.

[0048]The method 100 begins by first accessing (at 110) a set of
multicomponent marine calibration data exhibiting a plurality of
polarization vectors at each of a plurality of co-located pressure and
particle motion data points on a marine seismic survey apparatus. The
polarization vector polVec(t) is the vector composed of x, y, z
components of the measurement:

where t is the time, and x, y, and z are the inline, crossline, and
vertical directions that define a Cartesian coordinate system described
further below and vx, vy, vz denote the particle motion
measurements along the time, x, y, and z coordinates. Polarization angles
are the angles between the components of the polarization vector.

[0049]Note that for a plane wave, the polarization angles are constant and
independent of time. In other words if one plots the tip of the vector
polVec(t) as a function of t, it will stay on a line in a
three-dimensional ("3D") coordinate system. The angle that the line makes
with respect to the x, y, z axes of the 3D coordinate system will be the
same as polarization angles. For this type of a plane wave, it is said
that the wave (signal) is linearly polarized.

[0050]The method 110 then determines (at 120) a set of perturbations from
the calibration data 231 for the marine seismic survey apparatus from the
polarization vectors. The perturbation noise is the difference between
the recorded data in the absence of any perturbation errors and the
recorded data in the presence of sensitivity and alignment perturbation
errors. In the illustrated embodiment, the perturbation noise on the
calibration signal is related to the perturbations on the physical model
parameters such as sensor sensitivity deviation from the nominal
sensitivity and misalignment angle with respect to cable axis. The
components and the magnitude of the polarization vector are then related
to the acquired signal with particle motion and pressure sensors. The
signals measured by pressure and particle motion sensors are functions of
magnitude of the polarization vector and the angles between the
components of the polarization vector.

[0051]The method 100 then continues by determining (at 130) a set of
calibration values corresponding to the perturbations. Those skilled in
the art having the benefit of this disclosure will appreciate that the
nature of the calibrations will be dependent upon the nature of the
perturbation. Similarly, so will their application.

[0052]Those skilled in the art having the benefit of this disclosure will
also appreciate that, because of the source of the perturbation noise,
the calibration data and the seismic survey data should be acquired using
the same apparatus.

[0053]To further an understanding of the invention described above, one
particular embodiment of the invention manifesting several aspects
thereof will now be presented. Referring now to FIG. 3A-FIG. 3c, a towed
array, marine seismic survey apparatus 300 is shown. FIG. 3A is a
perspective view of the survey apparatus 300 deployed. FIG. 3B is a plan
view of the survey from the direction indicated by the arrow 303 in FIG.
3A--i.e., from astern and inline of the survey apparatus 300. FIG. 3c is
a plan view of the survey from the direction indicated by the arrow 306
in FIG. 3B--i.e., from the starboard, broadside.

[0054]A survey vessel 312 tows an array 315 of streamers 318 (only one
indicated) comprised of a variety of seismic sensor sondes 321 (only one
indicated). The instrumented sondes 321 house, in the illustrated
embodiment, a pressure sensor 400, a particle motion sensor 403, and an
orientation sensor 406, as is conceptually shown in FIG. 4.

[0055]The pressure sensor 400 may implemented using, e.g., a hydrophone
such as is known to the art. The pressure sensor 400 acquires the
"pressure data" 427 indicating the magnitude and time of arrival for
passing wavefronts in a conventional manner well known to the art. The
pressure sensor 400 may be any suitable pressure known to the art for
this purpose.

[0056]The particle motion sensor 403 measures not only the magnitude of
passing wavefronts, but also their direction. The particle motion sensor
403 may be implemented using, for example, at least two co-located
sensors in different (preferably orthogonal) directions in the plane
perpendicular to the inline axis of the streamer 318. Suitable particle
motion sensors are disclosed in U.S. application Ser. No. 10/792,511
(Publication No. 2005/0194201); U.S. application Ser. No. 10/233,266
(2004/0042341); and U.S. Letters Pat. No. 3,283,293. Thus, it would be
possible to determine the direction of propagation for wavefronts
detected by the pressure sensors 400.

[0057]Particle velocity is but one vector quantity associated with the
passing wavefront that may be used. Thus, in some embodiments, instead of
the particle velocity, the particle acceleration may be measured using a
suitable accelerometer. Suitable accelerometers include geophone
accelerometers ("GACs"), such as are commonly known in the art for use in
land-based seismic surveying, or micro electromechanical systems ("MEMS")
accelerometer. Suitable MEMS accelerometers are known to the art. For
example, MEMS accelerometers are disclosed in U.S. Letters Pat. No.
5,723,790; U.S. patent application Ser. No. 11/042,721 (Publication No.
2005/0160814); U.S. patent application Ser. No. 11/000,652 (Publication
No. 2005/0202585); and International Patent Application Serial No.
PCT/G2904/001036 (Publication No. WO 2004/081583). However, any suitable
accelerometer known to the art may be used.

[0058]The streamer 318 also provides a way to measure or detect the
orientation of the particle motion sensors 403 with respect to the
sea-surface or gravity field. This is the function of the orientation
sensor 406. The particle motion sensor 403 is ideally oriented to measure
in the "true" vertical direction. However, this is frequently not the
case, as the streamers 318 can rotate and twist during the seismic
survey. It is therefore desirable to know the true orientation of the
particle motion sensor 403 relative to the vertical so that a correction
may be effected during processing.

[0059]This may be done using inclinometers, for example. The inclinometer
may be a single and/or dual axis accelerometer formed on an integrated
circuit chip, such as the ADXL 103/203 single/dual axis accelerometer
produced by Analog Devices or that disclosed in U.S. application Ser. No.
10/623,904, entitled "Cable Motion Detection", filed Jul. 21, 2003, in
the name of Kenneth E. Welker and Nicolas Goujon, and commonly assigned
herewith. Alternatively, the DC component from MEMS sensors in
embodiments where MEMS sensors are used. Note that this means, in
embodiments using MEMS sensors, there may not be a separate orientation
sensor 406.

[0060]Some embodiments may employ additional sensors over and above those
shown. Some embodiments may employ another sensor to measure the inline
particle velocity. If another particle motion sensor measuring the inline
particle velocity is present an extra inclinometer measuring the inline
angle of the sensor with respect to the sea-surface is included.

[0061]In general, it is desirable for the measurements of the particle
motion sensors 403 be taken as close to the point the pressure data is
acquired by the pressure sensors 400 as is reasonably possible to reduce
pre-processing. However, it is not necessary that the particle motion
sensor 403 be positioned together with the pressure sensor 400 within the
sonde 321 as is the case for the illustrated embodiment. Thus, the
sensors 400, 403, and 406 are co-located on the streamer 318, e.g., they
are located within the same sonde 321. However, the sensors 400, 403, 406
need not be housed in the same sonde 321 to be co-located. Furthermore,
as will be discussed further below, it is not required that the sensors
400, 403, and 406 be co-located. Alternative embodiments may position the
particle motion sensors 403 on the streamer 318 without regard to the
positions of the pressure sensors 400, even to the extent that the two
groups of sensors may employ different inline spacings along the streamer
318. In these circumstances, vertical motion data or the pressure data
can be interpolated inline during processing using techniques known to
the art.

[0062]The sensors of the instrumented sondes 321 then transmit data
representative of the detected quantity over the electrical leads of the
streamer 318. The data from the pressure sensors 400, the particle motion
sensors 403, and the sensor orientation sensors 406 may be transmitted
over separate lines. However, this is not necessary to the practice of
the invention. Size, weight, and power constraints will typically make
separate lines undesirable. The data generated will therefore be
interleaved with the seismic data. Techniques for interleaving
information with this are known to the art. For instance, the two kinds
of data may be multiplexed. Any suitable technique for interleaving data
known to the art may be employed.

[0063]Referring now to FIG. 3A-FIG. 3c and FIG. 4, the data generated by
the sensors 400, 403, and 406 of the instrumented sondes 321 is
transmitted over the streamer 318 to a computing apparatus (not shown)
aboard the survey vessel 312. As those in the art will appreciate, a
variety of signals are transmitted up and down the streamer 318 during
the seismic survey. For instance, power is transmitted to the electronic
components (e.g., the pressure sensor 400 and particle motion sensor
403), control signals are sent to positioning elements (e.g., the
deflectors and birds as are known in the art, which are not shown), and
data is transmitted back to the survey vessel 312.

[0064]To this end, the streamer 318 provides a number of lines (i.e., a
power lead 409, a command and control line 412, and a data line 415) over
which these signals may be transmitted. Those in the art will further
appreciate that there are a number of techniques that may be employed
that may vary the number of lines used for this purpose. Furthermore, the
streamer 318 will also typically include other structures, such as
strengthening members (not shown), that are omitted for the sake of
clarity.

[0065]The spacing, dimensions, and positioning of the array 315 may be
implemented in accordance with conventional practice. For example, the
illustrated embodiment employs seven streamers 318, each of which
includes eight instrumented sondes 321. Those in the art having the
benefit of this disclosure will appreciate that the number of streamers
318 and the number of sondes 321 will be highly implementation specific.
Streamers 318, for instance, typically are several kilometers long, and
so there are considerably greater numbers of sondes 321 in a typical
towed array marine survey.

[0066]Returning now to FIG. 3A-FIG. 3c, a pair of source vessels
333a-333b, each of which tows a respective seismic source 336 are also
shown. The present invention employs two seismic source signals generated
from at least two different positions in order to obtain different
polarization vectors. The characteristics of the sources (e.g., frequency
content, strength, etc.) can be similar or different--this is immaterial
to the practice of the invention. The sources 336 are impulse type
sources and, more particularly, airguns such as are known in the art.
However, any suitable acoustic source may be used. Thus, in some
alternative embodiments, one or both of the sources 336 may be sweep
sources as are known in the art. The seismic signal emitted by the
sources 336 should have a high signal-to-noise ratio ("SNR").
Accordingly, some embodiments may reduce the tow speed of one or both of
the vessels 333a-333b to, for example, 3 knots to reduce the strength of
the interfering noise sources at high frequencies.

[0067]As those in the art will appreciate, the data collection through the
survey apparatus described above will typically be susceptible to errors,
or perturbations, in what may be called "physical model parameters".
There are many kinds of physical model parameters in the construction and
design of the survey apparatus 300 shown in FIG. 3A-FIG. 3c and described
above. Two common types of perturbation errors arise from sensor
misalignment and sensor sensitivities.

[0068]For example, the seismic data collected during a survey is typically
collected in a Cartesian coordinate system defined by orthogonal x-y-z
axes. The coordinate system used is illustrated in FIG. 5, and is defined
relative to the sensors 400, 403, 406, first shown in FIG. 4, orientation
within survey apparatus. More particularly, the x direction is "inline"
with the streamer 318, the y direction is "crossline" to the streamer,
and the z direction is vertical through the water column.

[0069]When the data is processed, the processing techniques generally
assume that the sensors are squared within this coordinate system. It
frequently happens, however, that the sensors are misaligned relative to
the axes as is shown in FIG. 6A-FIG. 6B and FIG. 7A-FIG. 7B. FIG. 6A-FIG.
6B illustrate an x-y misalignment in which FIG. 6A depicts a "true"
alignment and FIG. 6B depicts the misalignment. FIG. 7A-FIG. 7B depict a
y-z misalignment in which FIG. 7A depicts a "true" alignment and FIG. 7B
depicts the misalignment. As discussed above, multicomponent surveys
sense not only the arrival of passing pressure wavefronts, but also their
directions. This kind of misalignment error therefore causes errors in
the direction detection.

[0070]As another example, for some sensors, the sensitivity of the sensor
is not a constant but rather is a function of frequency. For these types
of sensors, the sensitivity estimation and correction should be done for
each frequency. For some other sensors, the response of the sensor to the
signal can be described by some mathematical equation (e.g., a frequency
selective filter defined in terms of resonance frequencies, and
amplitudes at resonance frequencies). For these types of sensors, the
response of the sensor to the signal at each frequency (i.e., the
sensitivity at that frequency) can be estimated as described in the
previous paragraph and then the resonance frequencies and the
corresponding amplitudes at resonance frequencies can be estimated if
needed.

[0071]The present invention therefore, in the illustrated embodiment,
acquires a set of seismic "calibration" data prior to or after the
conduct of the actual survey. This data is called "calibration data"
because it is used to estimate the perturbations, and the estimated
perturbations are used to mitigate noise in the acquired seismic data, as
opposed to being seismic survey data. The calibration data is acquired
using a seismic signal. The seismic sources 336 are triggered at
different times to impart acoustic signals from different depths 35 and
36 shown in FIG. 3c. The order in which the seismic sources 336 are
triggered is immaterial to the practice of the invention. Similarly, the
actual measures of the two depths 35 and 36 are not material so long as
they are different.

[0072]Calibration data collection from the starboard source 336 is
illustrated in FIG. 8. FIG. 8 shows the process at three points in time,
t0, t1, and t2. Those in the art will appreciate that the
survey apparatus 300 will be in motion and, unlike what is shown, its
position relative to the acoustic signals and the geological formation
will change over time.

[0073]At time t0, the starboard source 336 is triggered and imparts a
seismic signal 800 into the water column 803. At time t1, the
seismic signal 800 encounters a reflector 806, i.e., the interface
between the water column 803 and the seabed 809. A portion 812 of the
seismic signal 800 is reflected back to the survey apparatus 300 and a
portion 815 continues propagating. The portion 815 encounters a second
reflector 818, i.e., the interface between two layers 821, 822 in the
seabed 806, at time t2. A portion 824 is reflected back toward the
survey apparatus 300 and a portion 827 continues to propagate. This
continues until the propagating portions become too attenuated.

[0074]The reflected portions 812, 824, upon arrival at the survey
apparatus 300, are detected by the pressure and particle motion sensors
400, 403. The detected reflections are digitized and transmitted to a
data collection unit (not shown) aboard the survey vessel 312.

[0075]The process described for the starboard source 336 is then repeated
for the port source 336.

[0076]The marine seismic survey is then performed in accordance with
conventional practice. The seismic survey data resulting from the survey
is also digitized and transmitted to the data collection unit aboard the
survey vessel 312.

[0077]In the illustrated embodiment, the calibration data 231 is processed
to obtain the perturbation data 236. A user (not shown) invokes the
application 265 which then accesses the calibration data 231 and
processes it. Referring now to FIG. 1, the application 265:
[0078]accesses (at 110) a set of multicomponent calibration data
exhibiting a plurality of polarization vectors at each of a plurality of
co-located pressure and particle motion data points on a marine seismic
survey apparatus; [0079]determines (at 120) a set of perturbations for
the marine seismic survey apparatus from the polarization vectors; and
[0080]determines (at 130) a set of calibration values corresponding to
the perturbations.The resulting calibration values 237 are then stored.
The perturbations can then be corrected--e.g., by the application 265--to
calibrate the sensor responses.

[0081]As mentioned briefly above, the present invention employs co-located
pressure and particle motion data points. By using co-located (or
nearest) pressure measurements as reference, the technique constrains the
amplitude perturbations. Then, the direction of the source-to-receiver
azimuth vector is used to compute the alignment perturbation. Hence, with
the present invention, the amplitude and alignment perturbations are
solved in two separate steps.

[0082]The advantage of this technique is that it corrects perturbation of
sensors individually. In other words, unlike other, noise based
perturbation estimation and correction techniques, the array length need
not to be very long. The disadvantage is that, it requires one or more
source boats to generate seismic signal for each sensor station.

[0094]The linearly polarized velocity measurement can be described by the
following curve in the vy-vz plane:

v y cos f + v z sin f = P rc ( 3 )
##EQU00004##

wherein all quantities are as defined above. Note that, P can be measured
from a hydrophone sensor co-located with particle motion sensors; and f
can be measured by using, for instance, Global Positioning System ("GPS")
data. At this stage vy and vz are unknown true particle
velocity data.

[0095]Suppose that in local coordinates, we have the following perturbed
particle velocity measurements-- vy, vz and alignment angle f.
The unperturbed measurements in global coordinates would be:

Note that, in this equation, a, b, q are unknowns; and f, P, F are
measured quantities. We would like to select the values of the parameters
a, b, q such that, Eq. (6) turns into Eq. (3). In other words, we would
like to have

[0096]The problem is that there are infinitely many solutions for a, b and
q which would satisfy Eq. (7). One trivial solution is: a=0, b= P/(P sin
F) and q=f- f+p/2. Hence, one cannot estimate amplitude and alignment
perturbation simultaneously by using a single signal measurement.
Polarization measurements corresponding to two or more polarization
angles will fix this problem. For instance, if one measures polarization
corresponding to M sources, then a, b are found by solving

[0097]There is a special case where there is no amplitude perturbation.
Suppose that amplitude perturbations on particle motion sensors have been
corrected by using some alternative method; and the problem is estimation
of the alignment perturbation error and the sensitivity of the hydrophone
which is assumed to be uncalibrated. We assume that the amplitude
correction factor for the hydrophone is h. Then the set of equations
given by Eq. (8) simplifies to:

cos( f+q+F)=cos f (13)

sin( f+q+F)=sin f

P=hP

Hence, the alignment perturbation error is found as q=-( f+F), and the
hydrophone amplitude perturbation is found from the ratio of P and P.
Hence, in this case a single source measurement is enough to correct the
alignment perturbation on the particle motion sensors and the amplitude
perturbation on the hydrophone.

[0098]The present invention admits variation in the location of the
seismic sources 336, first shown in FIG. 3A-FIG. 3c, from which the
calibration data 231, first shown in FIG. 2, is acquired. In the
embodiments described above, the seismic sources 336 are located
broadside to the survey apparatus 300 on both the starboard and port
sides from two different source vessels 333a, 333b. Alternative
embodiments may, for example, use a single source vessel 333 suspending a
single seismic source 336 to depth 35 from the port side and then,
subsequently, to depth 36 on the starboard side. FIG. 9A-FIG. 9B
illustrate an alternative embodiment in which the two sources 336 are
suspended at two different depths 35, 36, from a single source vessel 333
broadside to the survey apparatus 300 on just one side--e.g., the
starboard side. Those in the art having the benefit of this disclosure
will appreciate still other variations on this theme.

[0099]Some embodiments may even use inline seismic sources 336 (only one
indicated), as shown in FIG. 10. Or, the seismic source used in
operation--i.e., during the survey--can generate the source signal. One
such embodiment is shown in FIG. 11. However, in these embodiments, the
cable 1100 for the seismic sources 336 is towed at a depth d deep enough
so that the time delay between the direct arrival 1110 and ghost
reflection 1120 is large enough that they can be distinguished. Note also
that the seismic source 336 will be used to acquire calibration data 231
from two different locations as was discussed above.

[0100]In the embodiments illustrated above, the co-located measurements
from which the perturbation noise is determined are acquired using
co-located sensors, such as is shown in FIG. 4. However, the invention is
not so limited. Some alternative embodiments may, for example,
interpolate data points from acquired data. Consider, for example, the
portion of the streamer 1200, shown in FIG. 12.

[0101]If the pressure sensors are not co-located with particle motion
sensors, the pressure measurements can be interpolated to the positions
of the particle motion sensor positions. In general, it is believed that
the interpolation of the pressure data to particle motion sensor
locations would be more accurate than using, for instance, the nearest
pressure measurement, for the determination of perturbation noise. The
interpolation is straightforward, especially when the pressure sensors
are uniformly spaced along the streamer 1200.

[0102]So, pressure sensors and particle motion sensors need not
necessarily be co-located in all embodiments. In FIG. 12, the sondes 1210
house both pressure sensors 400 and particle motion sensors 403, as shown
in FIG. 4. The remaining sondes 1220 (only one indicated) house only
pressure sensors 400. The pressure data acquired by the pressure sensors
400 of the sondes 1220 can then be interpolated, as conceptually
indicated by the broken arrows 1230 (only one indicated), to the position
of the nearest particle motion measurement location. The interpolation is
performed during processing. Note that a particle motion sensor 403 may
be paired with more than one or two pressure sensors in this manner.
Accordingly, the invention does not require that calibration data 231 be
acquired from co-located sensors, but only paired sensors and that the
pairing does not require a one-to-one correspondence.

[0103]The method of the present invention can also be applied to
multi-component seabed seismic data, as well. FIG. 13A-FIG. 13B depicts a
seabed survey 1300 in a stylized fashion. A plurality of ocean bottom
cables 1303 (only one indicated) each comprising a plurality of
instrumented sondes 330' (only one indicated), are positioned on the
seabed 1306. The sondes 330' collect data from reflections generated as
previously described and transmits it to the surface 1309. The data
collection, however, is subject to commonly observed "shear (noise) on
vertical".

[0104]Recall also that the acquired survey data 225 and calibration data
231 comprise pressure data 227, 232 and particle motion data 228, 233. In
the illustrated embodiments, the particle motion data 228, 233 that is
acquired is velocity data, or the particle displacement of the passing
wavefront. This is but one type of the particle motion data suitable for
use in the present invention. Alternative embodiments may, for instance,
acquire the acceleration of the passing wavefront, i.e., the second
derivative in time of the particle displacement. Other quantities may
also be suitable for use in the present invention, as well. Note further
that some embodiments may acquire one type of the particle motion data
and convert it in processing to use another. Thus, some embodiments might
acquire the velocity data, process it to take the time derivative, and
then use the acceleration data in the rest of the method of the
invention. Or, some embodiment might acquire the acceleration, integrate
it over time to get the velocity, and then use the velocity data in the
rest of the method.

[0105]The presently disclosed technique is not actually a part of the
marine seismic survey. It therefore can be done not only before the
survey as described above, but also in between or after the survey, too.
Generally, however, conducting this type of calibration should yield more
benefit if performed prior to the survey. In this situation, the
technique as described above can be generally summarize as follows:
[0106]acquired calibration data; [0107]estimate perturbations; and
[0108]determine calibration values corresponding to estimated
perturbations. (For instance if there is sensitivity perturbation, we
identify how much the sensitivity differs from the nominal sensitivity,
and we correct for this deviation when we acquired the seismic survey
data)During the survey, however, the technique can be summarized as
follows: [0109]acquire perturbed data [0110]correct response of the
sensors by using the estimated calibration values (correction of the
sensor response will remove the perturbation noise)However, in both
circumstance, techniques can be performed, e.g., in accordance with the
method 100 illustrated in FIG. 1.

[0111]Note that the data used for perturbation estimation is seismic data,
in the sense that, it is generated by, say, an airgun and propagates in
sea, etc. However this data is not the seismic survey data that will be
used for characterization of the earth layers, hydrocarbon locations
etc., in analyzing the underlying geological formation. Hence,
perturbation estimation can be thought as a preprocessing step before
starting to acquire the survey data although it need not necessarily be
performed pre-survey.

[0112]It is apparent from the above discussion that, in one aspect, the
present invention includes a computer-implemented method, such as the
method 100 of FIG. 1. In another aspect, the invention includes a
computing apparatus such as the computing apparatus 200 of FIG. 2,
programmed to perform such a method. In still another aspect, the
invention includes a program storage medium such as the optical disk 220,
encoded with instructions that, when executed by a computing apparatus,
performs a method such as the method 100.

[0113]Thus, some portions of the detailed descriptions herein are
presented in terms of a software implemented process involving symbolic
representations of operations on data bits within a memory in a computing
system or a computing device. These descriptions and representations are
the means used by those in the art to most effectively convey the
substance of their work to others skilled in the art. The process and
operation require physical manipulations of physical quantities. Usually,
though not necessarily, these quantities take the form of electrical,
magnetic, or optical signals capable of being stored, transferred,
combined, compared, and otherwise manipulated. It has proven convenient
at times, principally for reasons of common usage, to refer to these
signals as bits, values, elements, symbols, characters, terms, numbers,
or the like.

[0114]It should be borne in mind, however, that all of these and similar
terms are to be associated with the appropriate physical quantities and
are merely convenient labels applied to these quantities. Unless
specifically stated or otherwise as may be apparent, throughout the
present disclosure, these descriptions refer to the action and processes
of an electronic device, that manipulates and transforms data represented
as physical (electronic, magnetic, or optical) quantities within some
electronic device's storage into other data similarly represented as
physical quantities within the storage, or in transmission or display
devices. Exemplary of the terms denoting such a description are, without
limitation, the terms "processing," "computing," "calculating,"
"determining," "displaying," and the like.

[0115]Note also that the software implemented aspects of the invention are
typically encoded on some form of program storage medium or implemented
over some type of transmission medium. The program storage medium may be
magnetic (e.g., a floppy disk or a hard drive) or optical (e.g., a
compact disk read only memory, or "CD ROM"), and may be read only or
random access. Similarly, the transmission medium may be twisted wire
pairs, coaxial cable, optical fiber, or some other suitable transmission
medium known to the art. The invention is not limited by these aspects of
any given implementation.

[0125]The following patent application is incorporated by reference as if
set forth verbatim herein for their teachings regarding inclinometers:
[0126]U.S. application Ser. No. 10/623,904, entitled "Cable Motion
Detection", filed Jul. 21, 2003, in the name of Kenneth E. Welker and
Nicolas Goujon, and commonly assigned herewith.

[0127]The particular embodiments disclosed above are illustrative only, as
the invention may be modified and practiced in different but equivalent
manners apparent to those skilled in the art having the benefit of the
teachings herein. Furthermore, no limitations are intended to the details
of construction or design herein shown, other than as described in the
claims below. It is therefore evident that the particular embodiments
disclosed above may be altered or modified and all such variations are
considered within the scope of the invention. Accordingly, the protection
sought herein is as set forth in the claims below.